In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and lithographic printing ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is eventually transferred to the surface of a material upon which the image is to be reproduced.
Lithographic printing precursors useful for preparing lithographic printing plates or sleeves typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the precursor is considered as positive-working. Conversely, if the non-imaged regions are removed, the precursor is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging has become increasingly important in the printing industry. Lithographic printing precursors for the preparation of lithographic printing plates have been developed for use with infrared lasers that image in a platesetter in response to signals from a digital copy of the image in a computer. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the precursors.
Lithographic printing plates can contain a colorant (dye or pigment) in the radiation-sensitive composition that has the function of making the image visible so it can be evaluated by optical density measurements before being mounted on press. Such colorants provide contrast between the image and the background. Some lithographic printing precursors cannot contain a colorant. For example, imaged lithographic printing plate precursors that are usually developed on-press have a colorless coating because if a colorant is present, it could contaminate the lithographic printing ink and the fountain solution used for development and printing, with the result of altering the printed color shades. However, sometimes it is necessary for such lithographic printing plates to be used the same way as those developed off-press, and in such instances, the image must be seen to be evaluated.
Other lithographic printing precursors contain materials that are not compatible with contrast-providing colorants. Such lithographic printing precursors have a faint colored image that is difficult to distinguish from the anodized aluminum substrate background. This low image contrast makes it almost impossible to evaluate the resulting lithographic printing plates for image quality, such as image resolution as measured using optical density measurements before mounting the plates onto a press. Such “colorless” lithographic printing plates are also difficult to “register” (align) when mounting them onto a press. Image registration (alignment) is very important in color printing in order to ensure image sharpness (resolution) and correct tinting shades.
It is a well known phenomenon that offset lithographic printing members containing sufficient coloration to be visible and measureable, when subjected to a development step, can leave significant contaminants of the coloration dye within the developer unit and such coloration has been tolerated as an inevitable part of the process. While any resinous material left within the developer unit has to be removed, dye residues turn the entire developer colored and cannot be so easily removed. Thus, there is a need to provide color contrast in lithographic printing members without causing such problems.
U.S. Pat. No. 6,451,491 (Dhillon et al.) describes the high loading of contrast-providing pigments into the imaging layer of lithographic printing plates. Such high amounts of pigments can destabilize imaging chemistry or developers.
Other contrast-providing colorants for lithographic printing members are obtained from leuco dyes that become colored in the presence of an acid or thermal acid generator, as described for example, in U.S. Pat. No. 7,402,374 (Oohashi et al.), U.S. Pat. No. 7,425,406 (Oshima et al.), and U.S. Pat. No. 7,462,440 (Yamasaki). These lithographic printing members have some disadvantages. For example, the acid or radical forming mechanism can be triggered prematurely during the drying of the printing plate, leading to un-wanted color, especially in on-press developed printing plates. Alternatively, many times the colored form of the dye is a salt that is soluble in water, leading to excessive coating loss if the imaged printing precursor is developed in aqueous developer. In some instances when the coating and the color is washed off in developer, some of the colored forms of the dyes, especially the triarylmethane dyes are light sensitive and require additional precautions in handling the lithographic printing plates. Lastly, a high concentration of the leuco dye and the latent acid must be added to the coating to ensure a good contrast between the image and background (OD 0.8-1.00). These components adversely affect the shelf-life, mechanical properties, and run-length of the photosensitive coating.
In copending and commonly assigned U.S. Ser. No. 12/483,323 (filed Jun. 12, 2009 by Memetea, Huang, Munnelly, and Wertz), a method is described for treating a lithographic printing precursor having an optical density of OD1. After imaging, a coloring fluid is applied such that the optical density in the exposed regions is OD2 that is greater than OD1. The coloring fluid comprises a water-insoluble colorant and a solvent that is capable of swelling the exposed imageable layer.
An alternative approach is to add a dye such as Crystal Violet to the processing liquid (developer). While this does always provide the required effect, it causes considerable contamination of the processor.
Copending and commonly assigned U.S. Ser. No. 12/481,002 (filed Jun. 9, 2009 by Figov, Marom, and Levi) describes the use of spirolactams or spirolactones in the processing liquid (developer). These compounds are meant to be colorless in the liquid and to become colored when the pH is decreased.
There is a need for an improved means for providing contrast between the image and background of lithographic printing members, especially those prepared from negative-working lithographic printing precursors.